U.S. application Ser. No. 285,898, filed July 23, 1981, describes an encoded keyboard switch composed of four sections of flexible film. A first and second section have opposed surfaces on which there are first switch sites and the third and fourth sections also have opposed surfaces on which there are second switch sites. The switch sites are located such that a second switch site is in alignment with, and is behind, each first switch site so that when the first section is pressed at a given switch site, the first associated membrane switch will initially be closed and thereafter the second membrane switch will be closed. The conductors interconnect the switch sites in accordance with an encoding scheme which is such that a characteristic signal is produced in output conductors. Two common ground conductors are provided in accordance with the teachings of the application Ser. No. 285,898 in a manner such that when a switch site is depressed, no signals will be sent through the output conductor until connections are made to the ground conductors. This feature prevents the transmission of erroneous signals when a multi-bit signal is being transmitted, that is, a signal in two or more of the output conductors.
The present invention is directed to the achievement of an improved encoded switch of the general class described in application Ser. No. 285,898. Specifically, the invention is directed to the achievement of an encoded membrane switch having a reduced number of circuit conductors and with a reduced requirement in the number of switch electrodes is the encoding scheme. These reduced requirements result in a simplification of a circuitry of the switch (from the standpoint of the number of conductors required) with a resulting improvement in ease of manufacture and a corresponding reduction in manufacturing costs.
An encoded keyboard switch in accordance with the invention comprises a single sheet of flexible insulating material which has been folded along fold lines to produce a stack of four sections of film in parallel aligned relationship. The stack comprises a first section, a second second, a third section, and a fourth section. The first and second sections have opposed surfaces on which there are provided a plurality of first switch sites and the third and fourth sections have opposed surfaces on which there are provided a plurality of second switch sites, each first switch site being in alignment with a second switch site. First switch electrodes are provided on the opposed surfaces of the first and second sections at first switch sites and second switch electrodes are provided on the opposed surfaces of the third and fourth sections at section switch sites. A common ground conductor and a plurality of signal bit conductors are provided on the opposed surfaces with selected conductors extending across the folds of the single sheet. A tail extends from the sheet and the signal bit conductors and the ground conductor extend onto the tail. The switch assembly is characterized in that selected first switch sites and selected second switch sites are electrically isolated from the signal bit conductors and from the common ground conductor. Selected second switch electrodes on the second section are two pole electrodes and all switch electrodes on the first, third and fourth sections are single pole electrodes. All second switch electrodes on the third section and selected first switch electrodes on the first section are connected to the common ground conductor. The routing of the signal bit conductors is in accordance with an encoding scheme which produces a unique signal in the bit conductors on the tail when a specific switch site on the first section is depressed, the signal being a multi-bit signal and being produced in a plurality of signal bit conductors on the tail when some switch sites are depressed and being a single bit signal and being produced in a single bit conductor on the tail when other switch sites are depressed whereby, upon depression of the first section at a location above a predetermined first switch site, the opposed surfaces of the predetermined first switch site will be moved into contact with each other, and thereafter the opposed surfaces of the second switch site which is beneath the predetermined first switch site will be moved into contact with each other. A unique signal will then be produced by a resulting circuit extending from the ground conductor on the tail through signal bit conductors to at least one signal bit conductor on the tail. When the unique signal is a multi-bit signal, the circuit will not be completed by the ground conductor until all of the switch electrodes required for the production of the multi-bit signal have been electrically connected to each other.
In accordance with a further embodiment, the first section has tactile effect means thereon at each of the first switch sites, the tactile effect means comprising domes in the first section which are concave with respect to the surface of the second section. In accordance with a further embodiment, the single sheet has parallel side edges and end edges, the sheet being folded along fold lines which extend normally of the side edges and the tail extends from one of the end edges.
In accordance with a further embodiment, the second section is at one end of the sheet, the first section is beside the second section and the fourth section is between the first section and the third section with the tail extending from the second section. The sheet has a slit therein which extends along a fold line which is between the first section and the fourth section. The second section is folded against the first section and the tail is inserted through the slit. The third section is folded against the fourth section, and the third and fourth sections are folded as a unit against the first and second sections.